The present invention relates to the field of methods of measuring the properties of structures. The present invention relates more particularly to a method of characterizing a structure by means of an acoustic wave generated and detected by light pulse. The method uses measurement of the various reflections and propagations of the wave in the structure.
In the prior art, known U.S. Pat. No. 5,748,318 describes a system for the characterization of thin films and interfaces between thin films through measurements of their mechanical and thermal properties. In the system described, light is absorbed in a thin film or in a structure made up of several thin films, and the change in optical transmission or reflection is measured and analyzed. The change in transmission or reflection is used to supply information on the ultrasonic waves generated in the structure. In that way, it is possible to determine the thicknesses of the layers and various optical properties of the structure.
The above-mentioned patent is an example of implementation of a pump-probe system that is known to the person skilled in the art and that is described generally with reference to FIG. 1, which shows an example of a known device. In this figure, the light source is a short-pulse (e.g. femtosecond) laser emitting a wave of fixed wavelength generating a first beam that is split by a beam splitter into a “pump” beam and a “probe” beam. The optical path length of the “probe” beam is then caused to vary by means of a mirror that is position servo-controlled. It is then known that the properties of the structure under the effect of the emitted beams cause a change in the reflection (or transmission) properties of the probe wave. In particular, as shown in FIG. 2, also in a manner known per se, on a graph giving change in reflection as a function of time, it is possible to observe echoes characteristic of the interfaces of a structure. Analysis of the echo signal then makes it possible to deduce, for example, the thickness of the material, if the speed of propagation of the sound wave in the medium is known. However, using that method, it is not possible to determine both the speed of propagation and the thickness of the structure.
In order to increase the number of extracted characteristics, and in particular both the speed and the thickness, the publication entitled “Evidence of Laser-Wavelength Effect in Picosecond Ultrasonics: Possible Connection with Interband Transition” (Physics Review Letters, Mar. 12, 2001, Volume 86, Issue 12) describes the use of a pump-probe device as described above, but associated with a wavelength-tunable laser, thereby making it possible to cause the wavelength of the emitted signals to vary. By means of such wavelength effects, it is then possible to determine both thickness characteristics and speed characteristics for certain types of structure. As described in the publication entitled “A Novel Approach Using Picosecond Ultrasonics at Variable Laser-Wavelength for the Characterization of Aluminium Nitride Films Used for Microsystem Applications” (A. Devos, G. Caruyer, C. Zinck, and P. Ancey), World Congress on Ultrasonics (Paris Sep. 7-10, 2003), pp. 793-796 ISBN 2-9521105-0-6), for a structure that is transparent to the probe beam, an acousto-optical interaction appears in the material that causes oscillations to appear instead of mere pulses observed by echo. Such oscillations, referred to as “Brillouin” oscillations have a period dependent on the wavelength of the probe and on the speed of sound in the material. They are shown in FIG. 3 for two samples of SiN/Al/Si and SiO2/Al/Si. In that example, it can be understood that the materials and the thicknesses distinguish the two samples from each other so that the Brillouin oscillations do not have the same period at the same wavelength for the probe signal (430 nm). The person skilled in the art can understood that measuring the period of the Brillouin oscillations gives information on the speed of sound in the material, independently of the thickness of the layer. When the acoustic wave reaches the free surface, it reflects off it by changing the sign of the deformation. This results in a jump in reflectivity appearing. The acoustic wave generated in depth by the “pump” beam carries a minute change in the thickness of the layer whose sign changes on reflection. This change is detected optically because the transparent layer then acts as a Fabry-Perot interferometer, as shown in the publication entitled “Ultrafast Vibration and Laser Acoustics in Thin Transparent Films”, O. B. Wright and T. L Hyoguchi, Optics Letters, Vol. 16, page 1529 (1991).
Such jumps in reflectivity are shown in FIG. 3. It should be noted that the Brillouin oscillations can extend on either side of the jump in reflectivity. The person skilled in the art can then understand that measuring the appearance time of a jump gives information on the thickness of the material, while measuring the period of said oscillations gives information on the speed of propagation. For a material like AlN, thickness error rates of about 6% have been shown, by using the period of the oscillations and the position of an acoustic echo.
An object of the present invention is to reduce further the error rate on the measured data, while keeping the possibility of determining both thickness and speed values. The present invention thus intends to solve those prior art drawbacks by using, in particular, wavelength effects, e.g. by means of a tunable laser. To this end, the present invention is of the type described above and it is remarkable, in its broadest acceptation, in that it provides a device for characterizing a structure, said device comprising radiation generator means for generating a pump first radiation and a probe second radiation, said radiation generator means for generating said first and second radiations being suitable for delivering radiations at different wavelengths, time-shift generator means for generating a time shift between said probe second radiation and said pump first radiation at said structure, detector means for detecting said second beam after reflection off or transmission through said structure so as to generate a signal to be analyzed, and processor means for processing said signal, said device being characterized in that said processor means are suitable for identifying a zone corresponding to a jump in said signal, for determining the amplitude of said jump as a function of said different wavelengths, for comparing said amplitude with a theoretical model for variation of the amplitude as a function of wavelength, and for determining, for a wavelength that is characteristic of said theoretical model, a characterization value associated with the thickness of said structure and with the speed of propagation of radiation in said structure.
In an embodiment, in order to obtain a source whose wavelength can vary, at least one tunable laser source is used. In particular, it is possible to use two tunable laser sources, or indeed one fixed source and one tunable source. In another embodiment, the variation in wavelength is obtained by emitter means for emitting a continuum of light.
In order to make it possible to observe reflectivity jumps in accordance with the invention, said probe second radiation is chose to be suitable for interacting with at least two interfaces of layers of said structure. In order to ensure that the light signals are transmitted over a plurality of wavelengths, the device of the invention preferably further comprises a set of optical means adapted to transmit said radiations over a wavelength range corresponding to said different wavelengths.
The invention also provides a method of characterizing a structure, said method comprising the steps consisting in:                applying a pump first radiation to said structure;        applying a probe second radiation to said structure, said probe second radiation being time-shifted relative to the pump first radiation;        detecting said second radiation after reflection or transmission at said structure and generating a signal representative of said second radiation after reflection or transmission;        identifying an amplitude jump in said signal;        causing the wavelength of said second radiation to vary in a manner such as to obtain a first jump profile as a function of wavelength;        comparing said first profile with a theoretical second profile depending on wavelength, and on a function of the thickness and of the optical index of said structure; and        deducing therefrom a value associated with the thickness and with the optical index of said structure.        
For the purposes of the present Application, the term “jump” corresponds to an analysis zone presenting high variation in the mean value of reflectivity. The amplitude of a jump is then the difference in said mean values on either side of said zone. In the presence of Brillouin oscillations, the mean value is calculated over a length of time corresponding substantially to a Brillouin oscillation period.